Method of optimizing a GA—nitride device material structure for a frequency multiplication device

ABSTRACT

A preferred method of optimizing a Ga-nitride device material structure for a frequency multiplication device comprises:
         determining the amplitude and frequency of the input signal being multiplied in frequency;   providing a Ga-nitride region on a substrate;   determining the Al percentage composition and impurity doping in an AlGaN region positioned on the Ga-nitride region based upon the power level and waveform of the input signal and the desired frequency range in order to optimize power input/output efficiency; and   selecting an orientation of N-face polar GaN or Ga-face polar GaN material relative to the AlGaN/GaN interface so as to orient the face of the GaN so as to optimize charge at the AlGaN/GaN interface. A preferred embodiment comprises an anti-serial Schottky varactor comprising: two Schottky diodes in anti-serial connection; each comprising at least one GaN layer designed based upon doping and thickness to improve the conversion efficiency.

GOVERNMENT INTEREST

The embodiments herein may be manufactured, used, and/or licensed by orfor the United States Government without the payment of royaltiesthereon.

BACKGROUND OF THE INVENTION

A frequency multiplier (e.g. a tripler) is a device that, as an example,takes an input signal at 30 GHz and converts some of the energy fromthis input signal into an output signal at 90 GHz. Because a frequencymultiplier is used to generate an output at a different frequency fromthe input, one of the goals of an optimized structure is efficientoperation whereby the energy transferred to the new frequency at theoutput is maximized. The same methodology for the device/structure andoptimization can also be applied to obtain higher harmonics, such as thefifth order of conversion, which will be useful for sub-millimeter wavesignal generation.

Varactor multipliers have first been conceptualized back in the mid1960s. However, little has been done insofar as a conceptual method onhow to optimize the device-material structure for highest frequencyconversion efficiency at a particular output power level.

According to Wikipedia, varactors are operated in a reverse-biasedstate. No current flows, but since the thickness of the depletion zonevaries with the applied bias voltage, the capacitance of the diode canbe made to vary. Generally, the depletion region thickness isproportional to the square root of the applied voltage; capacitance isinversely proportional to the depletion region thickness. Thus, thecapacitance is inversely proportional to the square root of appliedvoltage. The depletion layer can also be made of a MOS or a Schottkydiode.

SUMMARY

A preferred embodiment of the present invention comprises asemi-insulator GaN (or SiC, or AN) substrate anti-serial Schottkyvaractor for efficient power-selective microwave to sub-millimeter wavefrequency signal multiplier involving varying the aluminum compositionof an AlGaN barrier layer to maximize, inter alia, output powerefficiency. In place of the AlGaN barrier, any semi-insulating layerwith energy gap greater than 3.5 and free carrier mobility less than 300cm2/V/s may be used. This list would include properly grown and dopeddiamond or boron-nitride appropriately grown for good crystallinequality on GaN. In a preferred methodology, selection of AlGaN/GaNvaractors containing either (1) a high-doped/low-doped GaN region or (2)just a low doped GaN region is utilized depending upon the amplitude ofthe input signal being tripled in frequency. Stronger susceptancemodulation is exhibited in AlGaN/GaN Anti-series Schottky Varactors(ASVs) made from Ga-face polar material compared to N-face polarmaterial. Results indicate that as a frequency tripler choosing theproper aluminum composition results in 27% conversion efficiency with aninput signal of 5 GHz and over 7% conversion efficiency with an inputsignal of 60 GHz along with optimization trends. With input voltageamplitudes over 10 V, an AlGaN/GaN structure with 15% Al providesgreater conversion efficiency than one with 5% Al. Power absorbed in thevaractor also increases as aluminum percent increases, affectingreliability and power transfer. Results of a GaN-based ASV performing asa frequency tripler for fundamental frequencies up to 110 GHz indicatean advantage to using an AlGaN/GaN epi-structure over only a GaNepi-structure.

A preferred method of optimizing a Ga-nitride device material structurefor a frequency multiplication device comprises:

determining the amplitude and frequency of the input signal beingmultiplied in frequency;

providing a Ga-nitride region on a substrate (GaN, or SiC, or AN);

determining the percentage composition of Al in an AlGaN region to bepositioned on the Ga-nitride region by selecting an aluminum compositionpercentage for doping based upon the desired frequency range for thefrequency multiplication device in order to optimize power input/outputefficiency; and

selecting an orientation of N-face polar GaN or Ga-face polar GaNmaterial relative to the AlGaN/GaN interface so as to orient the face ofthe GaN and to optimize charge at the AlGaN/GaN interface.

Optionally, the method may incorporate the selection of Ga-face polarmaterial as the N-face polar material creates a negative charge at theAlGaN/GaN barrier that repels electrons, and negatively impacts thesusceptance modulation as indicated by electron concentrations.

Optionally, the method may further comprise modeling the Ga-nitridematerial using drift-diffusion modeling based upon numerical simulationsto produce the capacitance-voltage curves when a predeterminedsinusoidal input voltage signal at a predetermined frequency in therange of 6 to 60 GHz is applied to the structure, and adjusting theimpurity doping of the AlGaN and GaN regions to optimize the nonlinearshape of the capacitance-voltage curve that is responsible for frequencymultiplication so that the V_(TR) (transition voltage) voltage, measuredat the point where the transition from high to low capacitance occurs inthe capacitance-voltage curve is optimal for an input RF signal of acertain power level, indicating peak voltage of input signal, wherebythe structure is optimized for the input power to be upconverted to ahigher frequency.

Optionally, the power input/output efficiency may be optimized dependentupon input voltage, and, for input voltages over 10 volts, an AlGaN/GaNstructure with approximately 15% Aluminum may be utilized to providegreater conversion efficiency than an AlGaN/GaN structure withapproximately 5% Aluminum.

Optionally one can make a tradeoff by determining if the efficiency gainfrom increasing the aluminum percent is a better outcome vs. as it mayalso lead to reduced lifetime due to heating of the device, but if thecost savings from increased efficiency is worth the reduced lifetime,this is a good result.

Optionally, the frequency multiplier is a varactor having an inputsignal, and the method further includes modeling based upon numericalsimulations to produce the capacitance-voltage curves, and wherein thealuminum composition of the AlGaN region and impurity doping of theAlGaN and GaN regions can be adjusted so that the voltage, V_(TR)measured where the transition from high to low capacitance occurs in thecapacitance-voltage curves, which is a point of nonlinearity in thecurves, is optimal for an input RF signal of a predetermined input powerlevel measured at peak voltage of an input signal.

Optionally, the method may comprise controlling the voltage, V_(TR),where the transition from high capacitance to low capacitance isoptimized by utilizing the intrinsic carrier concentration of thewide-bandgap material, AlGaN, which has a much higher intrinsic freecarrier concentration as compared to AlGaAs.

Optionally, the top of the AlGaN region is Ga-polar material which leadsto optimal spontaneous and piezoelectric polarization that producesoptimal electric fields for optimal performance.

Another preferred method for providing a varactor frequency multiplierstructure comprises:

providing a substrate and at least one semi-insulating region comprisingone or more of GaN, AlGaN, and impurity doped AlGaN;

modeling based upon numerical simulations to produce thecapacitance-voltage curves; the at least one semi-insulating regionbeing designed so as to optimize the transition voltage of thecapacitance-voltage curves produced by the modeling;

optimizing the frequency multiplier structure to increase the efficiencyof frequency multiplication by designing the structure for optimumperformance at a specific input power level; transition voltage of thecapacitance-voltage nonlinearity being based on the nonlinearitycapacitance-voltage curve of the device occurring at an optimum voltagefor a particular sinusoidal input signal being multiplied

Optionally, this alternative method may comprise a substrate having asemi-insulating GaN formed thereon, and wherein the at least onesemi-insulating region comprising one or more of GaN, AlGaN, andimpurity doped AlGaN forms parts of a pair of Schottky diodes that areanti-serial, with the anodes connected together, for odd-harmonicsgeneration.

Optionally, this alternative method may comprise, in order to minimizethe power absorbed in the varactor frequency multiplier structure sothat more power is transferred to the load, developing a phasedifference between the current and voltage waveforms at the varactorterminals so that one is at maximum when the other is close to zero.

Optionally, this alternative method may comprise maximizing the powerwhen frequency multiplying at the 3rd harmonic, and wherein the maximumpower transferred in the third harmonic drops off at a much greater ratefor larger output voltage V_(0-P), requiring proper modeling foraccurate design, because at the same time the varactor gets hotter asoutput voltage V0-P increases, and wherein the alternative methodcomprises tuning the nonlinearity in the capacitance voltage curve sothat the nonlinearity occurs at an optimum level to match the powerlevel of the input signal for maximum efficiency of power upconversion.

Optionally, using this alternative method, the frequency multiplierstructure may be an efficient high-power millimeter to sub-millimeterwave frequency generator and the at least one semi-insulating region maycomprise at least one semi-insulating wide bandgap AlGaN layer which isdesigned based on choice of composition, doping, and thickness togreatly improve the conversion efficiency, and the at least one GaNlayer may be designed based upon doping and thickness to greatly improvethe conversion efficiency.

A preferred embodiment of the present invention comprises two inanti-serial connection; each of the two inhomogeneously doped Schottkydiodes comprising at least one GaN layer which is designed based upondoping and thickness to greatly improve the conversion efficiency.

Optionally the preferred embodiment comprises two Schottky diodes thatare inhomogeneously doped and are in anti-serial connection, eachSchottky diode further comprising at least one semi-insulating widebandgap AlGaN layer which is designed based on choice of composition,doping, and thickness to greatly improve the conversion efficiency.

Optionally the preferred embodiment comprises modeling the varactorusing numerical simulations to produce the capacitance-voltage curveswhen a predetermined sinusoidal input voltage signal at a predeterminedfrequency in the range of 6 to 60 GHz is applied to the modeled varactorstructure, such that by adjusting the impurity doping of the AlGaN andGaN regions, the nonlinear shape of the capacitance-voltage curve thatis responsible for frequency multiplication is optimized and so that thetransition voltage, measured at the point where the transition from highto low capacitance occurs in the capacitance-voltage curve is optimalfor an input RF signal of a certain power level, indicating peak voltageof input signal, whereby the input power is optimized for optimalconsideration of the device nonlinearity so that efficient powerconversion occurs from the input frequency to the output frequency.

Optionally, the preferred embodiment comprises the at least onesemi-insulating wide bandgap AlGaN layer comprises incorporatingaluminum in the range of approximately 5% to 45% and wherein the voltageacross the varactor varies to an output voltage ranging fromapproximately 4 to 24 volts and wherein the output efficiency of thevaractor is maximized when the current and power are substantially outof phase.

Optionally the preferred embodiment comprises two anti-serial-Schottkydiodes which are quasi-monolithically integrated into microstriplinecircuit, and wherein the varactor is optimized for the highest frequencyconversion efficiency at a particular output power level by modifyingthe Aluminum content in the AlGaN layer.

Optionally the preferred embodiment comprises adjusting the impuritydoping and aluminum content of the AlGaN layer by modeling of thevaractor on a computer to produce capacitance-voltage curves; such thatthe aluminum composition of the AlGaN region and impurity doping of theAlGaN and GaN regions can be adjusted so that the voltage, V_(TR)measured where the transition from high to low capacitance occurs in thecapacitance-voltage curves is optimal for an input RF signal of apredetermined power level measured at peak voltage of an input signal.

Optionally the preferred embodiment comprises two inhomogeneously dopedSchottky diodes each comprise an ohmic contact through which an inputvoltage is inputted, and a Schottky contact, the Schottky contacts beingconnected together, and a surface passivation layer positioned betweenthe Schottky contacts and the ohmic contacts; and wherein each Schottkydiode comprises a GaN layer approximately 0.25 to 0.3 μm thick having acarrier concentration of approximately 1×10¹⁸ carriers per cm³, and anAlGaN layer approximately 0.02 μm thick having a carrier concentrationof approximately 5×10¹⁶ carriers per cm³.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, in which:

FIG. 1A is a schematic illustration of an equivalent circuit diagram ofa preferred embodiment representing the complete ASV structuresimulated.

FIG. 1B is a schematic illustration showing the cross sectionalstructure of one half of the ASV simulated. All dimensions, inmicrometers, are merely examples, and are approximate.

FIG. 2 is a schematic illustration showing the Epi-layer composition ofregions S1, S2 and S3 of the structure shown in FIG. 1.

FIG. 3A illustrates the concept of basing of the choice of structure oninput power. The semi-insulating region is designed so that the overlapof the signal with the capacitance-voltage curve is ideal for maximumconversion efficiency. The curves from left to right are for the AlGaNregion having an aluminum composition of 5%, 15%, 25%, 35%, and 45%.

FIG. 3B further illustrates the concept of FIG. 3A showing the signals1, 2, and 3 versus biasing voltage.

FIG. 3C is an illustration showing the capacitance-voltagecharacteristics at 60 GHz for structure S1. The curves with capacitancetransitions going from left to right in bias correspond to devices withAlGaN region aluminum percentages of 5%, 15%, 25%, 35%, and 45%.

FIG. 3D is an illustration showing the capacitance-voltagecharacteristics at 60 GHz for structure S2. The curves with capacitancetransitions going from left to right in bias correspond to devices withAlGaN region aluminum percentages of 5%, 15%, 25%, 35%, and 45%.

FIG. 4A is an illustration showing the power in the third harmonicrelative to total output power indicating optimum choice of structurebased on input signal voltage amplitude. The structure considered inFIG. 4A is S1 of FIG. 1, Aluminum composition of the AlGaN layers are 5%(solid line), 15% (dashed line), and 25% (dotted line). Input sinusoidalsignal's frequency is 60 GHz, and therefore, the output signal frequencybeing optimized for is at 180 GHz, example for 3^(rd) harmonicgeneration.

FIG. 4B is an illustration showing the Cmax/Cmin ratio measured acrossthe ohmic contacts for structure S2 at different frequencies. AlGaNregion aluminum compositions are 5% (solid line), 15% (dot dash) and 25%(dashed).

FIG. 5A illustrates power in the third harmonic relative to total poweracross the ASV for an input sinusoidal signal with zero to peakamplitude between 4 and 24 V at 5 GHz for structures S1 (curves withsolid markers), and S2. Aluminum compositions of the AlGaN layer/regionare 5% (solid line), 15% (dashed line), and 25% (dotted line).

FIG. 5B illustrates, power in the third harmonic relative to total poweracross the ASV for an input sinusoidal signal with zero to peakamplitude between 4 and 24 V at 60 GHz for structures S1 (curves withsolid markers), and S2. Aluminum compositions of the AlGaN layer/regionare 5% (solid line), 15% (dashed line), and 25% (dotted line).

FIG. 6 is an illustration showing current (dashed line) and voltage(solid line) across the ASV consisting of structure S2 with AlGaN layeraluminum composition of 15%, for the 60 GHz input sinusoid with voltagefrom 0 to peak, V_(0-P), of 16 V.

FIG. 7A illustrates the RF power absorbed per cycle for the structureanalyzed in FIG. 5A and also shows that the RF power absorbed per cyclesaturates (increases at a slower rate) as the signal amplitudeincreases.

FIG. 7B illustrates the RF power absorbed per cycle for the structureanalyzed in FIG. 5B, and also shows that the RF power absorbed per cyclesaturates (increases at a slower rate) as the signal amplitudeincreases.

FIG. 8A illustrates the electron concentration for an ASV containing thelayer structure S2 and with 15% Al composition at 15 V applied bias. Twodifferent cases of polarization charge at the Al₁₅Ga₈₅N/GaN interfaceare considered: positive for Ga-face polar material (solid curve) andnegative for N-face polar material (dashed curve). The gray regions arethe Schottky contact. In both FIGS. 8A and 8B, the Schottky contacts areconnected together. FIG. 8A illustrate a device to which a 15 V isapplied to the ohmic contact of the device

FIG. 8B also illustrates the electron concentration for an ASVcontaining the layer structure S2 and with 15% Al composition at 15 Vapplied bias. Two different cases of polarization charge at theAl₁₅Ga₈₅N/GaN interface are considered: positive for Ga-face polarmaterial (solid curve) and negative for N-face polar material (dashedcurve). The gray regions are the Schottky contact. In both FIGS. 8A and8B, the Schottky contacts are connected together. FIG. 8B illustrates adevice in which the ohmic contact is grounded. FIG. 1 indicates wherethe ohmic contact is relative to the Schottky contact for both FIGS. 8Aand 8B.

FIG. 9 illustrates results for an ASV based on structure S3 (representedin FIG. 2). Curves with markers indicate the power in the third harmonicrelative to total power (in current squared) across the ASV for an inputsinusoidal signal at frequency between 5 GHz and 110 GHz with V_(0-P) of4 V (dashed line), 8 V (solid line) and 16 V (dotted line). Total RFpower absorbed in the device per cycle is also indicated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the invention and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments that are illustrated in the accompanyingdrawings and detailed in the following description. It should be notedthat the features illustrated in the drawings are not drawn to scale.Descriptions of well-known components and processing techniques areomitted so as to not unnecessarily obscure the embodiments of theinvention. The examples used herein are intended merely to facilitate anunderstanding of ways in which the embodiments of the invention may bepracticed and to further enable those of skill in the art to practicethe embodiments of the invention. Accordingly, the examples should notbe construed as limiting the scope of the embodiments of the invention.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventionto those skilled in the art. In the drawings, the dimensions of objectsand regions may be exaggerated for clarity. Like numbers refer to likeelements throughout. As used herein the term “and/or” includes any andall combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the full scope of theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the full scope of theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various ranges, elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. Forexample, when referring first and second ranges, these terms are onlyused to distinguish one range from another range. Thus, a first element,component, region, layer or section discussed below could be termed asecond element, component, region, layer or section without departingfrom the teachings of the present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toother elements as illustrated in the Figures. It will be understood thatrelative terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the Figures. Forexample, if the device in the Figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower”, cantherefore, encompass both an orientation of “lower” and “upper,”depending of the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below. Furthermore, the term“outer” may be used to refer to a surface and/or layer that are farthestaway from a substrate.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

It will also be appreciated by those of skill in the art that referencesto a structure or feature that is disposed “adjacent” another featuremay have portions that overlap or underlie the adjacent feature.

A preferred methodology and embodiment comprises a varactor frequencymultiplier structure (approach applicable to higher odd-harmonicmultipliers) that can be optimized for highest efficiency frequencyconversion at a particular power level. Along with the methodology foroptimizing the semi-insulating/GaN structure, the present invention alsoprovides benefits coming from the wide bandgap semiconductor materialand high thermal conductive substrates which will allow for high poweroperation. Current varactor multipliers are based on InAs, GaAs, and InPmaterial systems; both the material and the currently used structuresput them at a disadvantage when it conversion efficiency and outputpower are considered.

The present invention is expected to have wide distribution inhigh-frequency, and high-power applications from personnel-sizedportable and ground based communication systems, radar and EW systems,satellite communications, IED detection and mitigation systems requiringRF sensing and RF countermeasures, biological/chemical hazard detection,and concealed weapons detection.

The article entitled “Critical Design Issues for High-power GaN/AlGaNAnti-serial Schottky Varactor Frequency Triplers”, by P. B. Shah and H.A. Hung, Microelectronics Journal, v. 43, p. 410, 2012, is herebyincorporated by reference.

The preferred embodiment of the invention includes AlGaN/GaN Schottkydiodes connected anti-serially so that their anodes are connectedtogether as shown in FIG. 1A. FIG. 1B shows one half of the invention orone Schottky diode 12. The two Schottky diodes 11, 12 are connected bythe Schottky contacts 17 form the preferred embodiment Anti-serialSchottky Varactor (ASV). The region 15 may comprise S1, or S2, or S3 ofFIG. 2. Shown in FIG. 1B is a region 16 having the epi-layer region 15,surface passivation Si₃N₄ layer 13, and Ohmic contact 14 formed thereon.Region 16 is preferably an epilayer of GaN grown or deposited on top ofa substrate (GaN, or SiC, or AN) and upon which regions 15, 14, and 13are placed. N_(D) is an example of the dopant concentration of donoratoms in the GaN layer to make it n-type. The dimensions shown in FIG.1B are approximate and are shown in micrometers. For example, the region15 may be approximately 0.3 microns thick, and approximately 7 micronsin length.

Drift-diffusion model based numerical simulations have produced thecapacitance-voltage curves shown in the top of FIG. 3 for structure S2when a sinusoidal input voltage signal at 60 GHz is applied to thedevice. The nonlinear shape of the capacitance-voltage curve isresponsible for frequency multiplication. The aluminum composition ofthe AlGaN region (shown within S1 and S2 in FIG. 2) and the impuritydoping of the AlGaN and GaN regions can be adjusted so that the voltage,V_(TR), where the transition from high to low capacitance occurs in thecapacitance-voltage curve (i.e. the nonlinearity in the curve) isoptimal for an input RF signal of a certain power level (peak voltage ofinput signal). This shift in V_(TR) doesn't occur in conventionalvaractor structures because the conventional varactor structures do notinclude semi-insulating materials. Therefore this optimization based oninput power cannot be achieved for conventional varactor structures.FIG. 3B illustrates three different voltage signals “signal 1”, “signal2”, and “signal 3.” The overlap of the input signal (S1, S2, or S3) withthe capacitance-voltage nonlinearity (“structure 2”, “structure 3”, or“structure 4”) is different. This needs to be considered analytically ornumerically for optimal consideration of the device nonlinearity so thatefficient power conversion occurs from the input frequency to the outputfrequency. FIG. 4A indicates that for a particular input power level(horizontal axis), the aluminum composition and doping concentration canbe properly chosen for optimum frequency conversion considering themethodology indicated in FIG. 3B.

Generally speaking, it is important that the top of the AlGaN region inFIGS. 1B and 2 should be Ga-polar material and not N-polar. The polaritycan be controlled by the growth method and substrate used. Bothmetal-organic chemical vapor deposition and molecular beam epitaxy canbe used to deposit the layers for the varactor structure. Designvariables are the aluminum composition of the AlGaN layer, and theintentional dopant level of both the AlGaN and GaN layers.

Presently known varactor multiplier structures cannot be as efficient infrequency conversion as an optimized version of the present invention,since they do not provide a method to optimize the structure so thatbased on the power of the input signal being converted to a higherfrequency, the output will contain most of the energy in the desiredharmonic. Prior art materials such as AlGaAs/GaAs varactor structures donot provide the means to control the voltage where the transition fromhigh capacitance to low capacitance occurs (the voltage where strongnonlinearity occurs) because the intrinsic carrier concentration of thewide-bandgap material (AlGaAs etc.) has a much higher intrinsic freecarrier concentration and mobility compared to AlGaN.

Also, the use of group III-nitride based varactors, because of theirhigher breakdown voltage and high thermal conductivity with the SiC orAN substrates, can offer much higher output power density thanstructures fabricated from GaAs or InP.

The present invention comprises a unique technique for optimizing thedevice structure to manufacture a highly efficient high-power millimeterto sub-millimeter wave frequency generator based on properly designingthe semi-insulating wide bandgap AlGaN layer (choice of composition,doping, and thickness), and the GaN layer (doping and thickness) togreatly improve the conversion efficiency for high frequency (harmonic)generation. This invention provides an additional degree of freedom fordesigning the frequency multiplier so that more power can be transferredfrom the input fundamental frequency to a higher harmonic frequency atthe output compared to the current state of the art.

The GaN based multiplier approach can offer much higher output power atmillimeter wave than other approaches based on fundamental frequencyamplifiers or sources.

Possible uses of the present invention include specific devicestructures that can provide high-frequency signals for efficienthigh-bandwidth soldier portable or ground based communication systems(wireless or satellite). Also, these devices can providesub-millimeter-wave sources for covert communications, astrophysics andplanetary science, sources for single frequency and hyper-spectraldetection of chemical or biological agents in the atmosphere.Furthermore, the devices can be used in systems that detect and mitigateor neutralize IEDs and also concealed weapon detection systems.Furthermore this can be used in electronic warfare systems.

The use of a semi-insulating material in conjunction with the preferredembodiments of the present invention provide another degree of freedomin designing the varactor structure for efficient frequency conversion(upconvert to third order or high order odd harmonics) at a particularinput signal power level of operation. Also, recognizing and takingadvantage of this degree of freedom to tune the nonlinearity in thecapacitance voltage curve so that the nonlinearity occurs at an optimumlevel to match the power level of the input signal for maximumefficiency of power upconversion.

The preferred embodiment semi-insulating/GaN/substrate anti-serialSchottky varactor frequency multiplier structure can be optimized toincrease the efficiency of frequency tripling (or higher ordermultiplication) by designing the structure for optimum performance at aspecific input power level. The preferred embodiment exhibits anadditional optimization degree of freedom (the transition voltage of thecapacitance-voltage nonlinearity) based on its semi-insulating regionscompositions (both the aluminum composition and impurity doping in thecase of AlGaN). These semi-insulating regions are to be designed so thatthe nonlinearity (transition) in the capacitance-voltage curve of thedevice occurs at an optimum voltage for a particular sinusoidal inputsignal being upconverted through frequency mixing.

The Group III-nitride materials used in conjunction with preferredembodiments of the present invention facilitate high-powerhigh-frequency applications, Group III-nitride materials offer theadvantage of high power operation due to their large energy gap (3.4 eVfor GaN and higher for AlGaN/GaN alloys) which leads to their highbreakdown voltage compared to other III-V compound semiconductormaterials. In addition, these III-nitride materials exhibit goodelectron saturation and peak velocities, large band offsets, and goodthermal conductivity. Cutoff frequencies of single diode GaN varactorshave been demonstrated at 360 GHz. See for example, C. Jin, et al., “Anovel GaN-based high frequency varactor diode,” Proceedings of the 5thEuropean Microwave Integrated Circuits Conference, 2010, pp. 118-121,hereby incorporated by reference. GaN based varactors are promising forhigh Q varactors with high operating power levels. See for example, W.Lu, et al., “InGaN/GaN Schottky diodes with enhanced voltage handlingcapability for varactor applications,” IEEE Electron Device Lett., 31(2010), pp. 1119-1121, hereby incorporated by reference.

To generate high frequencies, three different structures that can triplethe input frequency have been discussed in the literature for theIII-nitride material system. These are the metal-semiconductor-metal(MSM) 2D electron gas (2DEG) varactor (as described further in M. Marso,et al., “Comparison of AlGaN/GaN MSM varactor diodes based on HFET andMOSHFET layer structures” IEEE Electron Device Lett., 27 (2006), pp.945-947 (hereby incorporated by reference)), the anti-serial Schottkyvaractor (ASV), and the heterobarrier varactor (HBV), as describedfurther in M. Krach, et al. “Power generation at millimeter-wavefrequencies using GaAs/GaAlAs triplers,” Phys. Status Solidi (c). 1(2004), pp. 2160-2182, and M. Saglam, et al., “Influence of polarizationcharges in Al0.4Ga0.6N/GaN barrier varactors,” Appl. Phys. Lett., 82(2003), pp. 227-229, N. Tanuma, et al., “Capacitance analysis ofAl0.25Ga0.75N/GaN heterostructure barrier varactor diodes,” Phys. StatusSolidi (c), 2 (2005), pp. 2692-2695, all of which are herebyincorporated by reference.

The MSM-2DEG and anti-serial Schottky varactor (ASV) structures exhibitlower leakage currents due to the larger barrier a metal/semiconductoror metal/insulator/semiconductor structure presents to free carrier flowcompared to the AlGaN/GaN interface. On the other hand, both theanti-serial Schottky varactor (ASV) and HBV structures are typicallyconsidered vertical structures, while the MSM-2DEG structure is alateral structure. For GaN devices in general vertical structures canhandle higher power than lateral structures due to the electric fielddistribution. Besides these issues, symmetric performance is alsodesired for easier implementation and greater system reliability.

Symmetric varactors are a name given to varactors that offer symmetriccapacitance-voltage (CV) characteristics and anti-symmetriccurrent-voltage (I-V) characteristics. As varactors, these have anadvantage of not needing both a DC bias or idler circuits at evenharmonics when used as a frequency tripler. This is because the C-V andI-V characteristics of the structure results in an absence of secondharmonic generation and greater third harmonic conversion efficiency.The III-nitride materials are piezoelectric and this complicatescreating symmetric varactors using the HBV design because thepolarization fields lead to large sheet charge densities in the devicestructure that shift the axis of symmetry of the CV characteristics awayfrom zero bias. In this regard, see, O. Ambacher, et al., “Twodimensional electron gases induced by spontaneous and piezoelectricpolarization in undoped and doped AlGaN/GaN heterostructures,” J. Appl.Phys., 87 (2000), pp. 334-344, hereby incorporated by reference. Thepiezoelectric nature of the III-nitride materials can also lead todesign issues for anti-serial Schottky varactor (ASV) structures andthis will be described hereinafter.

The analytical results of the performance capabilities of AlGaN/GaN ASVfrequency triplers are demonstrated and some of the design issuesdiscussed hereinafter. Of the many device/material variables possible toinvestigate, the main focus is on AlGaN material composition, and howthis affects output power performance. However, this is merely anexample, and the invention is not limited to AlGaN material composition.

Procedure

The Anti-serial Schottky Varactor (ASV) is formed of two identicalSchottky diodes with their Schottky contacts connected together as shownin the schematic illustration given in FIG. 1A. Each of the Schottkydiodes 11, 12 comprises the layer structure shown in the FIG. 1B(although only one is shown in FIG. 1B. The input signal to be tripledis connected across the two ohmic contacts 14 of the ASV. FIG. 2 showsexamples of three different epi-layer compositions that can be used inconnection with the region 15 of FIG. 1B. However, the numbers ofdifferent epilayers increases as in the AlGaN layer, the Al compositioncan be varied, for example, from 5% to 45%. The Si₃N₄ used for thesurface passivation layer 13 may be 250 nm thick. As shown, the ASV iscomposed of high/low doped regions because a carefully designednon-uniform doping profile is utilized for susceptance modulation, evenwhen self biasing of the device takes place as had previously beendetailed. See in this regard M. Krach, et al., “Power generation atmillimeter-wave frequencies using GaAs/GaAlAs triplers,” Phys. StatusSolidi (c), 1 (2004), pp. 2160-2182. Results had been reportedpreviously for an AlGaAs/GaAs ASVs with the narrow energy bandgap regioncontaining a high/low doped layer structure (see for example, M. Krach,et al., An integrated ASV frequency tripler for millimeter-waveapplications, in: Proceedings of the 33rd European Microwave Conference,vol. 3, 2003, pp. 1279-1281) (hereby incorporated by reference).Similarly structure S1 contains the narrow energy bandgap GaN regionconsisting of a high/low doped region. On the other hand, for structureS2 the AlGaN layer can also be grown unintentionally doped with abackground concentration 1×10¹⁸ cm⁻³, and so this may be also chosen forthe high doped region (as described further in R. Oberhuber, et al.,“Mobility of two-dimensional electrons in AlGaN/GaN modulation-dopedfield-effect transistors, Appl Phys. Lett., 73 (1998), pp. 818-820(hereby incorporated by reference).

Cylindrical, three-dimensional structures formed by rotating thestructure in FIG. 1 about the left axis may be utilized and weresimulated using the drift-diffusion finite-element software packageAtlas/Blaze developed by Silvaco Inc. Conventional Ga-face polaritymaterial is considered for part of the study except towards the endwhere the performance issues with N-face polar material in an AlGaN/GaNASV structure are discussed (see in this regard O. Ambacher, et al.,“Two dimensional electrongases induced by spontaneous and piezoelectricpolarization in undoped and doped AlGaN/GaN heterostructures,” J. Appl.Phys. 87 (2000) 334-344. Polarization charges were included at theAlGaN/GaN and AlGaN/metal contact interfaces (see for example, V.Fiorentini, et al., “Evidence for nonlinear macroscopic polarization inIII-V nitride alloy heterostructures,” Appl. Phys. Lett. 80 (2002)1204-1206) (hereby incorporated by reference), along with thermionicemission. A theoretical value of the Richardson constant was used andscaled for different Al percentages (see for example, D. Qiao, et al.,“Dependence of Ni/AlGaN Schottky barrier height on Al mole fraction,” J.Appl. Phys. 87 (2000) 801-804. The Schottky metal/AlGaN barrier heightwas set to 1 eV to reasonably consider the large range of barrierheights reported in the literature. An appropriate concentration andfield dependent mobility model for GaN and AlGaN was used (see, forexample, M. Farahmand, et al., “Monte Carlo simulation of electrontransport in the III-Nitride Wurtzite phase materials system: binariesand ternaries,” IEEE Trans. Electron Devices 48 (2001) 535-542). Also asaturation drift velocity of 2.5×10⁷ cm/s was used for GaN (see, forexample, U. V. Bhapkar & M. S. Shur, “Monte Carlo calculation ofvelocity-field characteristics of Wurtzite GaN, J. Appl. Phys. 82 (1997)1649-1655 (hereby incorporated by reference)).

For all the simulations executed, it was assumed that the device isideally terminated at the input and output ports so that maximum powertransfer takes place from the source to the load and unwanted harmonicsare rejected. This allowed focus on designing a device for optimumgeneration of a given harmonic of the fundamental. Similar procedures ofanalyzing just the efficiency of the structure for frequencymultiplication by observing the ratio J3/J1 was done to understandnarrower energy bandgap material varactor performance (see for example,in this regard, A. Reklaitis, “Efficient heterostructure doped-barriervaractor diodes,” J. Appl. Phys. 105 (2009) 024502-1-024502-5 (herebyincorporated by reference)) and for THz generation in doubleheterojunction structures (see for example, in this regard, D. S. Ong &H. L. Hartnagel, “Enhanced THz frequency multiplier efficiency byquasi-ballistic electron reflection in double-heterojunction structures,Euro-phys. Lett. 81 (2008) 48004-1-48004-6 (hereby incorporated byreference)).

Initially, AC simulations were done with the device simulation softwarepackage Silvaco, Atlas/Blaze, to obtain the capacitance-voltage (C-V)curves of the Anti-serial Schottky Varactor (ASV). Following that,transient simulations of the ASV were performed with an input voltagesine wave followed by taking a Fourier transform of the resulting outputcurrent to obtain the frequency spectrum. The ASV's third harmonicconversion capability is determined as the amplitude squared of thethird harmonic current through the ASV divided by the amplitude squaredof the total current through the ASV. An integral in time of the productof the total current through the ASV and voltage across provided thepower absorbed in the device over a cycle.

Results

FIG. 3C illustrates the 60 GHz small signal capacitance across the ASVfor structures S1 and FIG. 3D illustrates the 60 GHz small signalcapacitance across the ASV for structure S2 as the bias is increased.These curves are important since small signal analysis of varactors canbe used to understand their large signal performance as a frequencytripler (on this point, see for example, Y. Morandini, et al.,“Characterization of MOS varactor with Large Signal Network Analyser(LSNA) in CMOS 65 nm bulk and SOI technologies, in: Proceedings of the69^(th) ARFTG Conference, vol. 1, 2007, pp. 1-4) (hereby incorporated byreference). To measure these types of curves, one would need to do acomplete calibration and proper lumped element modeling of the deviceaccounting for parasitic, as is typically done using a network analyzerto obtain S parameter models of HEMTs (on this point, see for example,A. Jarndal & G. Kompa, “A new small signal modeling approach applied toGaN devices,” IEEE Trans. Microwave Theory Tech. 53 (2005) 3440-3448(hereby incorporated by reference)). FIGS. 3C, 3D indicate that as thepercent of aluminum increases, the knee voltage for the capacitancetransition from C_(max) to C_(min) moves to higher applied biases. Thisis because the wider bandgap AlGaN region has a lower intrinsic freecarrier concentration making it more resistive than the GaN region andtherefore a larger fraction of the applied bias drops across it ratherthan the GaN region. With larger Al composition the region becomes evenmore resistive, moving the knee further to the right of FIGS. 3C, 3D.Eventually the GaN region starts to deplete and that produces most ofthe capacitance drop observed. FIGS. 3C, 3D also indicates that thecapacitance drop off is steeper for the varactor with an AlGaN/low dopedGaN (S2) compared to the AlGaN/high/low doped GaN (S1) layer structure.This is also connected with the lower intrinsic free electronconcentration in the AlGaN region compared to the GaN region.Considering this concentration difference, charge neutrality requiresthat more of the applied bias drops across the AlGaN layer for S1compared to S2, because S1 has a higher doped region next to the AlGaNlayer compared to S2. Therefore, with less voltage dropping across theGaN region closest to the AlGaN layer in S1 compared to S2, as theapplied bias increases, less charge is depleted from the high doped GaNregion of S1 and therefore the drop in capacitance vs. voltage is lessfor S1 than that for S2 as shown in FIG. 3. The high doped GaN region ofS1 is fully depleted, the rate of change of capacitance with voltage isroughly the same for both S1 and S2 as observed for capacitance valuesbelow 25 pF, because the charge is being removed from regions of thesame (low) dopant concentration.

Furthermore, also observed in FIGS. 3C & 3D is that C_(max) and C_(min)of the curves differ only slightly; this difference being directlyrelated to the charge introduced by the polarization and intrinsic freecarrier concentration of the larger energy gap material. Though in thesefigures only positive voltage values are shown, simulations indicate thecurves are symmetric about V=0. The model used here had been verified byreproducing correctly the measured C-V curves of AlGaN/GaN Schottkydiodes reported in the literature, as for example, in W. L. Liu, et al.“Capacitance-voltage spectroscopy of trapping states in GaN/AlGaNheterostructure field-effect transistors,” J. Nanoelectron.Optoelectron. 1 (2006) 258-263, hereby incorporated by reference.

One of the figures of merit for a varactor is the dynamic cutofffrequency, calculated as f_(C)=(½πR_(S))×(1/C_(min)−1/C_(max)) where Cis capacitance and R_(S) is the series resistance. See in this regard,M. Krach, J. Freyer, & M. Claassen, “Power generation at millimeter-wavefrequencies using GaAs/GaAlAs triplers,” Phys. Status Solidi (c)1 (2004)2160-2182, and D. Choudhury, et al., “Study of the effect of theCmax/Cmin Ratio on the Performance of Back-to-Back Barrier-N-N (bbBNN)varactor frequency multipliers,” IEEE Microwave Guided Wave Lett. 4(1994) 101-103. Series resistance results from the aggregate ofspreading resistance, contact resistance and the impedance of undepletedregions; all of which are exhibited in the structures considered.Assuming similar series resistance is present in the device structuresinvestigated, the capacitance ratios C_(max)/C_(min) may be compared inFIG. 4B. There C_(max)/C_(min) over an input signal frequency range from0 to 100 GHz for structure S2 is presented with aluminum percentages of5%, 15% and 25%. That is, the AlGaN region with aluminum compositions of5%, 15% and 25% are shown by the solid, dot-dash and dashed lines,respectively. The capacitance ratio is limited at higher frequencies bythe total time for charge transfer from the reverse bias state to thezero bias condition of the Schottky diode. These times include thecharge carrier recombination times, the dielectric relaxation time, andthe mobility of the free carriers. At higher frequencies thesetime-limited effects affect device performance.

For the Al_(0.25)Ga_(0.75)N region structure simulated in FIG. 4B,plotting free carriers around the GaN/AlGaN junction indicates that at20 GHz the depletion region width is 0.23 μm and at higher frequencies,the distribution of charges in this region decreases. The slightdifference between the curves in FIG. 4B for the different aluminumcompositions (seen up to 35 GHz) is due to the added effects ofpolarization charges modifying the actual charge carrier concentrationin the GaN region, and the resistance of the AlGaN layer. At frequenciesabove 35 GHz, the change in the charge density in the AlGaN region isgreater than the change in the GaN region and thus, the AlGaN region'scharge modulation also has an influence on the capacitance ratio plottedat higher frequencies. These trends are very similar for structure S1.For these devices, the conduction current was less than 1 nA.

FIGS. 5A and 5B illustrate power in the third harmonic relative to totalpower across the ASV for an input sinusoidal signal with zero to peakamplitude between 4 and 24 V at 5 GHz, (FIG. 5A), and 60 GHz (FIG. 5B),for structures S1 (curves with solid markers), and S2. Aluminumcompositions of the AlGaN layer are 5% (solid line), 15% (dashed line),and 25% (dotted line). FIGS. 5A and 5B further indicate the efficiencyfor frequency multiplication at the 3rd harmonic of structures S1 and S2for an input signal of 5 GHz (FIG. 5A), and 60 GHz (FIG. 5B). In thesefigures, the input signal amplitude V_(0-P) increases from 4 to 24 V.For both S1 and S2, 5%, 15%, and 25% Al content in the AlGaN layer areconsidered. These results indicate that with higher Al percent, thepeak, I₃ ²/I_(TOTAL) ², shifts to larger input signal amplitudes. Thiscan be understood by comparing these results to FIGS. 3C and 3D. There,the knee voltage for the capacitance transition moves to higher biasesas the aluminum composition increases. It was concluded that there isoptimum input sinusoidal voltage amplitude whereby the interaction ofthe input sinusoidal voltage with the capacitance-voltage characteristictakes greatest advantage of the nonlinearity present in the capacitanceto generate a strong third harmonic in the resulting current. If theapplied voltage signal is of higher or lower amplitude such that theexcursion in the input voltage signal does not overlap well with thetransition from C_(max) to C_(min) in the capacitance-voltagecharacteristic, the overall effect of the nonlinear capacitance-voltagecharacteristic would be less than the maximum possible 3rd harmonicresponse. Also, it was observed that the peaks for structures S1 and S2occur at different amplitudes of the input signal, V_(0-P). This isrelated to the steeper transition from C_(max) to C_(min) seen forstructure S2 compared to structure S1 in FIGS. 3C and 3D. Finally, itwas observed that the peaks in FIGS. 5A and 5B shift to higher V_(0-P)for larger aluminum compositions, and this is directly related to thefact that the corresponding knee voltage for the capacitance transitionin FIGS. 3C and 3D also moves to the right. The results shown in FIGS.5A and 5B indicate that when tripling the frequency of a largeramplitude signal, one should pay particular attention to the Alcomposition of the AlGaN layer.

Besides increasing the cutoff frequency, it is desirable to minimize thepower absorbed in the varactor so that more power is transferred to theload. This is brought about by obtaining a phase difference between thecurrent and voltage waveforms at the varactor terminals so that one isat maximum when the other is close to zero. These waveforms are shown inFIG. 6 for structure S2 with 15% Al in the AlGaN layer and a 60 GHzinput voltage with V_(0-P)=16 V. FIGS. 7A and 7B show the power absorbedper cycle for the same structures analyzed in FIGS. 5A and 5B,respectively. It is interesting to observe here that while theefficiency for frequency multiplication at the 3rd harmonic peaks andthen decreases as V_(0-P) increases, the power dissipated in thevaractor continues to increase. Thus, the maximum power transferred inthe third harmonic drops off at a much greater rate for larger V_(0-P),while at the same time the varactor gets hotter as V_(0-P) increases.

FIGS. 7A and 7B also show that the power absorbed per cycle saturates asthe signal amplitude increases. Furthermore, comparing FIGS. 7A and 7Bone can see that at higher frequency more power is absorbed per cycle.This can be understood considering each Schottky diode varactor as beingmodeled by a series combination of a resistor and a capacitor. As thefrequency increases, the impedance of the capacitance decreases in thisseries combination due to greater charge transferred per unit time, andtherefore more power is dissipated in the resistive portion of thevaractor. This is also expressed with the quality factor, Q, for thevaractor when modeled using a series combination of a resistor, R_(S),and a capacitor, C_(S). The quality factor, Q=1/ωR_(S)C_(S), decreasesat higher frequencies and therefore more power is dissipated in thevaractor and bounding the upper frequency limit of operation.

The AlGaN/GaN structures simulated up to this point are made of Ga-facepolar material. The term Ga-face polar or the other common equivalent,N-face polar, refers to the arrangement of the atoms in the crystal.This arrangement leads to spontaneous and piezoelectric polarizationthat causes electric fields in the device which also influenceperformance. Ga-face polar material is being challenged in highfrequency RF MMIC applications by N-face polar AlGaN/GaN structures forHEMTs because of their potentially better properties for high frequencyoperation. See, in this regard, Rajan, et al., “N-polar GaN/AlGaN/GaNhigh electron mobility transistors,” J. Appl. Phys. 102 (2007)044501-1-044501-6.

However, N-face polar material is not advantageous for this particularASV structure where analysis indicates very low conversion efficiencies.The reason for this is the negative charge at the AlGaN/GaN barrierinterface when using N-face polar material. This negative charge repelselectrons and thus negatively impacts the susceptance modulation asindicated by the electron concentrations shown in FIGS. 8A, the lefthalf, and 8B, the right half of the ASV structure. These figures presentthe electron concentration through a vertical cut near the Schottkycontact at x=2 μm from the center of the circular ASV structure S2 thatwas simulated with an AlGaN layer containing 15% Al. For this result 15V is applied to the ohmic contact of FIG. 8A while grounding the ohmiccontact of FIG. 8B.

Also simulated was a GaN only ASV structure S3. However, it was firstobserved that the models produced results comparable to data measuredfor GaN varactors published recently, thus verifying the models we areusing. See in this regard, C. Jin, et al. “A novel GaN-based highfrequency varactor diode,” in: Proceedings of the 5^(th) EuropeanMicrowave Integrated Circuits Conference, 2010, pp. 118-121 and M.Saglam, et al., Influence of polarization charges in Al0.4Ga0.6N/GaNbarrier varactors, Appl. Phys. Lett. 82 (2003) 227-229. AC simulationsover the frequency range from 5 GHz to 100 GHz indicate that C_(max) islarger for lower frequencies while C_(min) remains the same. Also,unlike the AlGaN/GaN case, the transition voltage from C_(max) toC_(min) stays the same over this frequency range. The C_(max)/C_(min)ratios at different frequencies were 6.7 at 5 GHz, 5.3 at 30 GHz, 3.3 at60 GHz, and 1.8 at 100 GHz. The frequency tripling capability of thisdevice is presented in FIG. 9. There we show power in the third harmonicrelative to total power in the device vs. input signal frequency for aninput amplitude of V_(0-P)=4 V (dashed line), 8 V (solid line) and 16 V(dotted line). Also shown are the powers absorbed over one cycle in thisdevice for the same input signal amplitudes indicating saturation of theabsorbed power as the frequency increases. One can observe in thisfigure that at low frequencies the conversion efficiencies for 3rdharmonic generation for the 4 V and 8 V cases are similar and this isdue to similar voltage range over which C_(max) occurs unlike theAlGaN/GaN structures. The conversion efficiency for the 16 V inputsignal is very low, because the interaction between the input sinusoidalvoltage signal and the nonlinearity in the capacitance-voltagecharacteristic is non-optimal, with most of the excursion of the voltagesignal being outside of the nonlinearity in the capacitance-voltagecharacteristic.

Drift diffusion simulation results have been presented for AlGaN/GaN ASVas well as only a GaN ASV. For AlGaN/GaN varactors, it has beendemonstrated that the Al composition is a key design variable inoptimizing the performance. Also, AlGaN/GaN varactors containing either(1) a high doped/low doped GaN region, or (2) just a low doped GaNregion have been compared demonstrating that the choice of whichstructure to use also depends on the input signal amplitude. Theadvantages of using Ga-face polar compared to N-face polar AlGaN/GaNmaterial for ASVs has been demonstrated. Finally, results for a GaN ASVperforming as a frequency tripler for fundamental frequencies up to 110GHz indicate an advantage to using an AlGaN/GaN structure over only aGaN structure due to the additional degree of optimization allowed bythe AlGaN layer.

As used herein, ASV or anti-serial Schottky varactor means a varactorstructure comprising two inhomogeneously doped Schottky diodes inanti-serial connection which may be quasi-monolithically integrated intomicrostripline circuit (which may be on quartz).

As used herein, the terminology Ga-face polar, or the other commonequivalent, N-face polar, refers to the arrangement of the atoms in thecrystal.

The terminology Fermi level as used herein relates to dopedsemiconductors, p-type and n-type, where the Fermi level is shifted bythe dopant, illustrated by their band gaps. The Fermi function f(E)gives the probability that a given available electron energy state willbe occupied at a given temperature. The Fermi function is defined as:

${f(E)} = \frac{1}{{\mathbb{e}}^{{({E - E_{F}})}/{kT}} + 1}$Where E is the band gap of the material, E_(F) is the Fermi energy inthe band gap, k is Boltzmann's constant=8.6×10⁻⁵ eV/K, and T is thetemperature in Kelvin. The Fermi function dictates that at ordinarytemperatures, most of the levels up to the Fermi level E_(F) are filled,and relatively few electrons have energies above the Fermi level.

As used herein, the term “light” means electromagnetic radiation, unlessspecifically pointed out to the contrary. That is, the photodetectorsillustrated in the figures herein may be used in conjunction with otherforms of electromagnetic radiation and the terminology “light”encompasses other forms of electromagnetic radiation.

As used herein the terminology “processor” includes computer,controller, CPU, microprocessor; multiprocessor, minicomputer, mainframe, personal computer, PC, signal processor, super computer,coprocessor, and combinations thereof or any machine similar to acomputer or processor which is capable of processing algorithms.

The terminology “layer” as used in the following claims is not intendedto be limiting; including as to size or dimension. The terminology“layer” means a quantity or thickness of material, which may or may notbe of uniform thickness or have uniform dimensions. The terminology“layer” as used in the claims includes regions or portions of acomposite device or structure.

As used herein the terminology “energy gap” or “band gap” means anenergy range in a semiconductor where no electron states exist and whichmay be represented graphically as the energy difference (in electronvolts) between the top of the valence band and the bottom of theconduction band in thesemiconductor material.

As used herein the terminology “mobility” means the mobility of chargecarriers in a semiconductor material. The charge carriers may be eitherelectrons or holes.

Electron mobility refers to how quickly an electron can move through ametal or semiconductor, when pulled by an electric field. Insemiconductors, there is an analogous quantity for holes, called holemobility. When an electric field E is applied across a semiconductormaterial, the electrons respond by moving with an average velocitycalled the drift velocity.

As used herein the terminology “susceptance” means is the imaginary partof a complex number value of admittance. Admittance is a measure of howeasily a circuit will allow current to flow.

As used herein the Ga face polar GaN refers to GaN in a wurtzite crystalstructure wherein the four tetrahedrally oriented bonds of Ga areoriented in a way such that one bond points up and is not bonded to anitrogen atom but instead forming the exposed face of the crystal andthe other three bonds are downward and bonded to nitrogen atoms.

As used herein the N face polar GaN refers to GaN in a wurtzite crystalstructure wherein the four tetrahedrally oriented bonds of nitrogen (N)are oriented in a way such that one bond points up and is not bonded toa Ga atom but instead forming the exposed face of the crystal and theother threebonds are downward and bonded to Ga atoms.

The foregoing description of the specific embodiments reveal the generalnature of the embodiments herein and the present invention is notlimited to the embodiments disclosed. Applying current knowledge,modifications and/or adaptations of such specific embodiments arecontemplated to be within the scope of the present invention, and,therefore, such adaptations and modifications should and are intended tobe comprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology employed herein is for the purpose of description and not oflimitation. Therefore, while the embodiments herein have been describedin terms of preferred embodiments, those skilled in the art willrecognize that the embodiments herein can be practiced with modificationwithin the spirit and scope of the appended claims.

The invention claimed is:
 1. A method of optimizing a Ga-nitride devicematerial structure for a frequency multiplication device comprising thefollowing steps not necessarily in sequential order: determining theamplitude and frequency of the input signal being multiplied infrequency; providing a substrate; providing a Ga-nitride region on a thesubstrate; determining a percentage composition of Al in an AlGaN regionto be positioned on the Ga-nitride region by selecting an aluminumcomposition percentage and doping based upon the desired frequency rangefor the frequency multiplication device in order to optimize powerinput/output efficiency; and selecting an orientation of N-face polarGaN or Ga-face polar GaN material relative to the AlGaN/GaN interface soas to orient the face of the GaN so as to optimize charge at theAlGaN/GaN interface.
 2. The method of claim 1 wherein the substrate isone of SiC, GaN, or AN and wherein the Ga-face polar material selectedas the N-face polar material creates a negative charge at the AlGaN/GaNbarrier that repels electrons and negatively impacts the susceptancemodulation as indicated by electron concentrations.
 3. The method ofclaim 1 further comprising modeling the Ga-nitride material on acomputer using drift-diffusion modeling based upon numerical simulationsto produce the capacitance-voltage curves when a predeterminedsinusoidal input voltage signal at a predetermined frequency in therange of 6 to 240 GHz is applied to the structure, and adjustingimpurity doping and aluminum composition of AlGaN and GaN regions tooptimize the nonlinear shape of the capacitance-voltage curve that isresponsible for frequency multiplication so that the transition voltage,measured at the point where the transition from high to low capacitanceoccurs in the capacitance-voltage curve, is optimal for an input RFsignal of a certain power level, indicating peak voltage of inputsignal, whereby the structure is optimized for the input power to beupconverted to a higher frequency.
 4. The method of claim 3 wherein theadjusting the impurity doping and aluminum composition of the AlGaNregion and GaN regions is optimized by modeling a predeterminedsinusoidal input voltage on a computer to determine that the maximum ofpredetermined sinusoidal input voltage is within the capacitance-voltagecurve of the structure.
 5. The method of claim 1 wherein the powerinput/output efficiency is optimized dependent upon input voltage andwherein for input voltages over 10 volts an AlGaN/GaN structure withapproximately 15% Aluminum provides greater conversion efficiency thanan AlGaN/GaN structure with approximately 5% Aluminum.
 6. The method ofclaim 1 wherein by varying the Aluminum from 5% to 15% increases theamount of power absorbed in the varactor and increases its reliabilityand power transfer.
 7. The method of claim 1 wherein the frequencymultiplication device is a varactor having an input signal, and furtherincluding modeling based upon numerical simulations to produce thecapacitance-voltage curves, and wherein the aluminum composition of theAlGaN region and impurity doping of the AlGaN and GaN regions can beadjusted so that the transition voltage measured where the transitionfrom high to low capacitance occurs in the capacitance-voltage curves,which is a point of nonlinearity in the curves, is optimal for an inputRF signal of a predetermined power level measured at peak voltage of aninput signal.
 8. The method of claim 7 further comprising controllingthe transition voltage where the transition from high capacitance to lowcapacitance is optimized by utilizing the intrinsic carrierconcentration of the wide-bandgap material AlGaN which has a much higherintrinsic free carrier concentration as compared to AlGaAs.
 9. Themethod of claim 1 wherein the top of the AlGaN region is Ga-polarmaterial which leads to optimal spontaneous and piezoelectricpolarization that produces optimal electric fields for optimalperformance.